JEE Mains Chemistry · Biomolecules

Amino Acids: Essential, Codes and Properties

The twenty α-amino acids differ only in their side chain, which fixes each one's one-letter code, its class (acidic, basic or neutral) and the test that picks it out; ten are essential, all but glycine are chiral, and as dipolar ions they are high-melting, water-soluble solids.

All Biomolecules notes JEE Mains Chemistry strategy

Why this matters

Nineteen PYQs, seventeen multiple choice and two asking for a number, five from 2026. Eight test the one-letter codes, which amino acids are essential, and which side chains are acidic, basic or contain sulphur. Seven are about the structure itself: chirality, the dipolar ion and its properties, counting atoms, and cysteine and tyrosine. Four match a side-chain group to the test that detects it, or ask what ninhydrin reacts with.

Concept 1 of 3: The twenty amino acids: codes, side chains and the essential ones

Every amino acid in a protein is

\mathrm{H_2N{-}CH(R){-}COOH}

; only R changes. Essential amino acids are the ones the body cannot make, so they must come from food. One-letter codes use the first letter where it is free (G, A, V, L, I, S, T, C, M, P, H); where it is taken, the code is another letter, and those are the ones that are asked.

Definition

Essential (the ten NCERT marks): valine, leucine, isoleucine, arginine, lysine, threonine, methionine, phenylalanine, tryptophan, histidine. Arginine and histidine are called semi-essential in some books, but NCERT counts them as essential.
Non-essential: glycine, alanine, serine, cysteine, aspartic acid, glutamic acid, asparagine, glutamine, proline, tyrosine.
Class by the number of acid and amino groups: equal numbers → neutral; more COOH → acidic (aspartic, glutamic acid); more basic N groups → basic (lysine, arginine, histidine). The amide N of asparagine and glutamine is not basic, so those two are neutral.
Sulphur: cysteine ( $\mathrm{-CH_2SH}$ ) and methionine ( $\mathrm{-CH_2CH_2SCH_3}$ ).
Rings: histidine has an imidazole ring, tryptophan an indole ring, and proline's side chain closes back onto its own N to make a five-membered ring.
Codes that are asked: D aspartic acid, E glutamic acid, N asparagine, Q glutamine, K lysine, R arginine, F phenylalanine, W tryptophan, Y tyrosine.

Amino acid	Code	Side chain R	Class	Essential?
Glycine	G	$\mathrm{-H}$	Neutral	No
Alanine	A	$\mathrm{-CH_3}$	Neutral	No
Valine	V	$\mathrm{-CH(CH_3)_2}$	Neutral	Yes
Leucine	L	$\mathrm{-CH_2CH(CH_3)_2}$	Neutral	Yes
Isoleucine	I	$\mathrm{-CH(CH_3)CH_2CH_3}$	Neutral	Yes
Arginine	R	$\mathrm{-(CH_2)_3NHC(=NH)NH_2}$	Basic	Yes
Lysine	K	$\mathrm{-(CH_2)_4NH_2}$	Basic	Yes
Glutamic acid	E	$\mathrm{-CH_2CH_2COOH}$	Acidic	No
Aspartic acid	D	$\mathrm{-CH_2COOH}$	Acidic	No
Glutamine	Q	$\mathrm{-CH_2CH_2CONH_2}$	Neutral	No
Asparagine	N	$\mathrm{-CH_2CONH_2}$	Neutral	No Its amide N is not basic, so asparagine has only one basic group, the α-NH₂.
Threonine	T	$\mathrm{-CH(OH)CH_3}$	Neutral	Yes
Serine	S	$\mathrm{-CH_2OH}$	Neutral	No
Cysteine	C	$\mathrm{-CH_2SH}$	Neutral	No
Methionine	M	$\mathrm{-CH_2CH_2SCH_3}$	Neutral	Yes
Phenylalanine	F	$\mathrm{-CH_2C_6H_5}$	Neutral	Yes
Tyrosine	Y	$\mathrm{-CH_2C_6H_4OH}$ (para)	Neutral	No
Tryptophan	W	$\mathrm{-CH_2}$ -indolyl, two fused rings, one with N	Neutral	Yes
Histidine	H	$\mathrm{-CH_2}$ -imidazolyl, a five-membered ring with two N	Basic	Yes
Proline	P	$\mathrm{-CH_2CH_2CH_2-}$ joined back to the α-N, a five-membered ring	Neutral	No

Ten are essential. The letters that are not the first letter of the name are D, E, N, Q, K, R, F, W and Y.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 4 Apr 2026 Shift 1 · Q44Moderate

Example 1 · Biomolecules · Amino Acids: Essential, Codes and Properties

Match the LIST-I with LIST-II :

	List-I Name of amino acid		List-II One letter symbol/type
(A)	Arginine	(I)	D / Non-essential
(B)	Aspartic acid	(II)	R / Essential
(C)	Lysine	(III)	E / Non-essential
(D)	Glutamic acid	(IV)	K/Essential

Choose the correct answer from the options given below:

Aspartic acid is D, asparagine is N

A is alanine, so neither Asp nor Asn gets it. The acid takes D and the amide takes N; glutamic acid is E and glutamine is Q in the same way.

Tyrosine is not essential

The body makes tyrosine from phenylalanine, so tyrosine is non-essential while phenylalanine is essential. Proline is non-essential too.

Proline's ring is five-membered, and histidine has a ring

Proline's ring holds four carbons and the N. Histidine carries an imidazole ring, a heterocycle, so a statement that histidine has no heterocyclic ring is false.

Drill 7 more on the twenty amino acids: codes, side chains and the essential ones

Concept 2 of 3: Structure, dipolar ion and chirality of α-amino acids

An amino acid carries an acid group and a basic group on the same carbon. In the solid and in water the COOH gives its proton to the NH₂, and the molecule becomes a dipolar ion (zwitterion). That makes amino acids behave like salts: crystalline, high-melting, soluble in water and not in benzene. The α-carbon carries four different groups in every amino acid except glycine, so all the others are chiral.

Definition

Hydrolysis of proteins gives α-amino acids only: the $\mathrm{NH_2}$ is on the carbon next to the COOH.
Dipolar ion: $\mathrm{H_3\overset{+}{N}{-}CH(R){-}COO^-}$ . Amino acids are colourless crystalline solids, fairly high melting, water soluble, insoluble in non-polar solvents such as benzene, and amphoteric. Each ionisable group has its own $pK_a$ , so an amino acid has more than one.
Chirality: except glycine (R = H), all naturally occurring α-amino acids are optically active, and most have the L configuration. Threonine and isoleucine have two stereocentres, since their side chains carry one more.
Aspartic acid and glutamic acid both carry a COOH in the side chain.
Cysteine's SH is easily oxidised: two cysteines join through an $\mathrm{-S{-}S-}$ bridge to give cystine, the same disulphide link that holds protein chains.
Thyroxine, the thyroid hormone, is an iodinated derivative of tyrosine (Y).
To count atoms, add the backbone $\mathrm{C_2H_4NO_2}$ (as $\mathrm{H_2N{-}CH{-}COOH}$ ) to the side chain R.

An α-amino acid and its dipolar ion

\mathrm{H_2N{-}CH(R){-}COOH} \;\rightleftharpoons\; \mathrm{H_3\overset{+}{N}{-}CH(R){-}COO^-}

Worked example

Methionine has the side chain

\mathrm{-CH_2CH_2SCH_3}

. Find its molecular formula, the number of carbon atoms and the number of stereocentres.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 2 Apr 2025 · Q37Moderate

Example 2 · Biomolecules · Amino Acids: Essential, Codes and Properties

Identify the correct statement among the following:

Not every chiral amino acid has one stereocentre

Threonine and isoleucine each have two. A statement that all naturally occurring amino acids except glycine have exactly one chiral centre is false.

Amino acids are salts, not organic liquids

As dipolar ions they are crystalline, high-melting and water soluble, and they do not dissolve in benzene. A statement that arginine is highly soluble in benzene is the false one.

Protein hydrolysis gives α-amino acids

Every amino acid from a protein has its NH₂ on the α-carbon. Options with β, γ or δ amino acids describe no natural protein.

Drill 6 more on structure, dipolar ion and chirality of α-amino acids

Concept 3 of 3: Tests for amino-acid side chains and the ninhydrin test

These questions borrow tests from other chapters. Find the functional group in the side chain, then recall which test that group answers: a phenol gives a violet colour with neutral ferric chloride, an alcohol a red colour with ceric ammonium nitrate, a primary amine reacts with Hinsberg's reagent, and a primary amide undergoes Hoffmann bromamide degradation. Ninhydrin, by contrast, reacts with the α-amino acid part that every amino acid and protein shares.

Definition

NCERT's Biomolecules chapter does not list these tests. They come from the Alcohols, Phenols and Ethers chapter (FeCl₃, ceric ammonium nitrate) and the Amines chapter (Hinsberg's reagent, Hoffmann bromamide); here they only ask you to spot the group in the side chain.
Ninhydrin gives a purple colour with α-amino acids, peptides and proteins such as egg albumin. Starch, cellulose and PVC have no amino group and give nothing. The purple product is called Ruhemann's purple; its structure is outside NCERT.
Proteins with aromatic side chains (tyrosine, tryptophan, phenylalanine) also give the yellow xanthoproteic test with concentrated nitric acid.

Amino acid	Side-chain group	Test	Result
Tyrosine	Phenolic OH	Neutral FeCl₃	Violet colour
Serine, threonine	Alcoholic OH	Ceric ammonium nitrate	Red colour
Lysine	Primary amine, $\mathrm{-NH_2}$	Hinsberg's reagent, $\mathrm{C_6H_5SO_2Cl}$	Sulphonamide that dissolves in alkali
Glutamine, asparagine	Primary amide, $\mathrm{-CONH_2}$	Hoffmann bromamide, $\mathrm{Br_2}$ with NaOH	Amine with one carbon fewer
Tyrosine, tryptophan, phenylalanine	Benzene ring	Xanthoproteic, concentrated $\mathrm{HNO_3}$	Yellow colour
Every α-amino acid and protein	Free α-amino group	Ninhydrin	Purple colour

Match the group in the side chain first; the test follows from the group.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 6 Apr 2026 Shift 1 · Q45Moderate

Example 3 · Biomolecules · Amino Acids: Essential, Codes and Properties

Match List-I with List-II

	List-I Amino acid		List-II Positive reaction/Test for functional group present in side chain of amino acid
(A)	Glutamine	(I)	Hinsberg's test
(B)	Lysine	(II)	Neutral ${FeCl}_{3}$ test
(C)	Tyrosine	(III)	Ceric ammonium nitrate test
(D)	Serine	(IV)	Hoffmann bromamide degradation

Choose the correct answer from the options given below :

Lysine carries an amine, glutamine an amide

Lysine's side chain ends in NH₂ on a CH₂, a primary amine, so it answers Hinsberg's test. Glutamine's ends in CONH₂, a primary amide, so it undergoes Hoffmann bromamide degradation.

Ferric chloride needs a phenol

Serine and threonine have alcoholic OH and give no FeCl₃ colour. Only tyrosine, with its phenolic OH, gives the violet colour.

Drill 3 more on tests for amino-acid side chains and the ninhydrin test

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Structure, dipolar ion and chirality of α-amino acids
An α-amino acid and its dipolar ion
$\mathrm{H_2N{-}CH(R){-}COOH} \;\rightleftharpoons\; \mathrm{H_3\overset{+}{N}{-}CH(R){-}COO^-}$

Reference tables (2)

The twenty amino acids: codes, side chains and the essential ones20 rows

Amino acid	Code	Side chain R	Class	Essential?
Glycine	G	$\mathrm{-H}$	Neutral	No
Alanine	A	$\mathrm{-CH_3}$	Neutral	No
Valine	V	$\mathrm{-CH(CH_3)_2}$	Neutral	Yes
Leucine	L	$\mathrm{-CH_2CH(CH_3)_2}$	Neutral	Yes
Isoleucine	I	$\mathrm{-CH(CH_3)CH_2CH_3}$	Neutral	Yes
Arginine	R	$\mathrm{-(CH_2)_3NHC(=NH)NH_2}$	Basic	Yes
Lysine	K	$\mathrm{-(CH_2)_4NH_2}$	Basic	Yes
Glutamic acid	E	$\mathrm{-CH_2CH_2COOH}$	Acidic	No
Aspartic acid	D	$\mathrm{-CH_2COOH}$	Acidic	No
Glutamine	Q	$\mathrm{-CH_2CH_2CONH_2}$	Neutral	No
Asparagine	N	$\mathrm{-CH_2CONH_2}$	Neutral	No Its amide N is not basic, so asparagine has only one basic group, the α-NH₂.
Threonine	T	$\mathrm{-CH(OH)CH_3}$	Neutral	Yes
Serine	S	$\mathrm{-CH_2OH}$	Neutral	No
Cysteine	C	$\mathrm{-CH_2SH}$	Neutral	No
Methionine	M	$\mathrm{-CH_2CH_2SCH_3}$	Neutral	Yes
Phenylalanine	F	$\mathrm{-CH_2C_6H_5}$	Neutral	Yes
Tyrosine	Y	$\mathrm{-CH_2C_6H_4OH}$ (para)	Neutral	No
Tryptophan	W	$\mathrm{-CH_2}$ -indolyl, two fused rings, one with N	Neutral	Yes
Histidine	H	$\mathrm{-CH_2}$ -imidazolyl, a five-membered ring with two N	Basic	Yes
Proline	P	$\mathrm{-CH_2CH_2CH_2-}$ joined back to the α-N, a five-membered ring	Neutral	No

Ten are essential. The letters that are not the first letter of the name are D, E, N, Q, K, R, F, W and Y.

Tests for amino-acid side chains and the ninhydrin test6 rows

Amino acid	Side-chain group	Test	Result
Tyrosine	Phenolic OH	Neutral FeCl₃	Violet colour
Serine, threonine	Alcoholic OH	Ceric ammonium nitrate	Red colour
Lysine	Primary amine, $\mathrm{-NH_2}$	Hinsberg's reagent, $\mathrm{C_6H_5SO_2Cl}$	Sulphonamide that dissolves in alkali
Glutamine, asparagine	Primary amide, $\mathrm{-CONH_2}$	Hoffmann bromamide, $\mathrm{Br_2}$ with NaOH	Amine with one carbon fewer
Tyrosine, tryptophan, phenylalanine	Benzene ring	Xanthoproteic, concentrated $\mathrm{HNO_3}$	Yellow colour
Every α-amino acid and protein	Free α-amino group	Ninhydrin	Purple colour

Match the group in the side chain first; the test follows from the group.

Watch out for (8)

Aspartic acid is D, asparagine is N
→ The twenty amino acids: codes, side chains and the essential ones
Tyrosine is not essential
→ The twenty amino acids: codes, side chains and the essential ones
Proline's ring is five-membered, and histidine has a ring
→ The twenty amino acids: codes, side chains and the essential ones
Not every chiral amino acid has one stereocentre
→ Structure, dipolar ion and chirality of α-amino acids
Amino acids are salts, not organic liquids
→ Structure, dipolar ion and chirality of α-amino acids
Protein hydrolysis gives α-amino acids
→ Structure, dipolar ion and chirality of α-amino acids
Lysine carries an amine, glutamine an amide
→ Tests for amino-acid side chains and the ninhydrin test
Ferric chloride needs a phenol
→ Tests for amino-acid side chains and the ninhydrin test

Test yourself on Biomolecules

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